1. Field of the Invention
This invention relates to an improvement in a coolant temperature control system of an internal combustion engine for the purpose of controlling coolant temperature at suitable levels even at a high load engine operating range.
2. Description of the Prior Art
Most internal combustion engines of automotive vehicles and the like are provided with an engine cooling system using an engine coolant which is circulated through a coolant jacket formed in an engine body, in which the temperature of the coolant is controlled in response to engine load conditions so as to be at relatively high levels during a low load engine operating range and to be at relatively low levels during a high load engine operation in order to achieve improvements in fuel economy, exhaust emission (particularly hydrocarbons emission reduction), and power output.
Such coolant temperature control has been carried out, for example, under the action of a coolant temperature control system which includes a thermostat disposed in a coolant passageway leading to a radiator. The thermostat is operatively connected to a diaphragm actuator which controls the valve opening temperature of the thermostat in response to engine load condition.
However, the thermostat of such an coolant temperature control system is so arranged that the discharge pressure of a coolant pump acts in a direction to close a valve member of the thermostat. Consequently, the valve opening temperature of the thermostat is unavoidably shifted to a higher temperature side as engine speed increases to raise the revolution speed of a coolant pump. As a result, the coolant temperature control is carried out within higher levels at a high speed engine operating range even in case where engine load is constant. This deteriorates the durability the engine and shortens the life of the same.